Functional Description
Table 16-13. Packet Descriptor Word 4 (PD4) Bit Field Descriptions
Bits
Field Name
Description
The buffer pointer is the byte-aligned memory address of the buffer
31-0
Buffer 0 pointer
associated with the buffer descriptor. The CPU initializes this field for
transmitted packets; the DMA overwrites this field on packet reception.
Table 16-14. Packet Descriptor Word 5 (PD5) Bit Field Descriptions
Bits
Field Name
Description
The 32-bit word aligned memory address of the next buffer descriptor in
the packet. If the value of this pointer is zero, then the current buffer is the
31-0
Next descriptor pointer
last buffer in the packet. The CPU initializes this field for transmitted
packets; the DMA overwrites this field on packet reception.
Table 16-15. Packet Descriptor Word 6 (PD6) Bit Field Descriptions
Bits
Field Name
Description
31-22
Reserved
Reserved
The buffer length field indicates the original size of the buffer in bytes.
This value is not overwritten during reception. This value is read by the Rx
DMA to determine the actual buffer size as allocated by the CPU at
21-0
Original buffer 0 length
initialization. Since the buffer length in Word 3 is overwritten by the Rx
port during reception, this field is necessary to permanently store the
buffer size information.
Table 16-16. Packet Descriptor Word 7 (PD7) Bit Field Descriptions
Bits
Field Name
Description
31-22
Reserved
Reserved
The buffer pointer is the byte-aligned memory address of the buffer
associated with the buffer descriptor. This value is not overwritten during
reception. This value is read by the Rx DMA to determine the actual
21-0
Original buffer 0 pointer
buffer location as allocated by the CPU at initialization. Since the buffer
pointer in Word 4 is overwritten by the Rx port during reception, this field
is necessary to permanently store the buffer pointer information.
16.3.9.2.2 Host Buffer Descriptor/Buffer Descriptor (BD)
The buffer descriptor is identical in size and organization to a Packet Descriptor but does not include valid
information in the packet level fields and does not include a populated region for protocol specific
information. The packet level fields are not needed since the SOP descriptor contains this information and
additional copy of this data is not needed/necessary.
Host buffer descriptors are designed to be linked onto a host packet descriptor or another host buffer
descriptor to provide support for unlimited scatter / gather type operations. Host buffer descriptors provide
information about a single corresponding data buffer. Every host buffer descriptor stores the following
information:
•
Pointer to the first valid byte in the data
•
Length of the data buffer
•
Pointer to the next buffer descriptor in the packet
Buffer descriptors always contain 32 bytes of required information. Since it is a requirement that it is
possible to convert a descriptor between a Buffer Descriptor and a Packet Descriptor (by filling in the
appropriate fields) in practice, Buffer Descriptors will be allocated using the same sizes as Packet
Descriptors. In addition, since the 5 LSBs of the Descriptor Pointers are used in CPPI 4.1 for the purpose
of indicating the length of the descriptor, the minimum size of a descriptor is always 32 bytes
1738
Universal Serial Bus (USB)
SPRUH73H – October 2011 – Revised April 2013
Copyright © 2011–2013, Texas Instruments Incorporated